I also use an old car battery. I went out to the bin of used batterties at work and tested a bunch until i found a good one. A lot of guys at the field use a small 12V 35Ah deep cycle battery. It is a lot lighter to carry around.

You can also buy a 12V power supply at Radio Shack or other electronics stores but good ones can get expensive.

Hobby City has osme power supplies too:

http://www.hobbycity.com/hobbycity/s...y_for_Chargers

http://www.hobbycity.com/hobbycity/s...100~240V_12.5A

http://www.hobbycity.com/hobbycity/s...ly_100~240v_5A

If you buy from Hobby City, the shipping from Hong Kong can be pretty high on just one item. It pays to order a number of items because the shipping is usually the same.